Nested neutron spectrometer

Abstract: Nested Neutron Spectrometers T M (NNS) can be used to measure neutron fluence rate spectra under diverse circumstances with a working principle similar to Bonner sphere systems. Conventionally, the NNS consists of an active-readout He-3 detector core and concentric moderator shells. In environments where the neutron fluence rate exceeds ∽ 104 neutrons/s, these spectrometers may be operated in a current-mode to avoid the effects of pulse pile-up and deadtime. A current-to-pulse conversion factor is used to convert current-mode measurements to pulse-mode. However, the conversion factor can only be directly calibrated under low-flux conditions due to the pulse pile-up in high-flux situations. In order to have confidence in the use of the conversion factor in high neutron fluence rate environments such as in high-energy radiotherapy, its use must be experimentally validated. To perform this validation, we developed a passive-readout NNS with gold activation foils. Our work included the generation of system response functions using the Monte Carlo toolkit, GEANT4, and an experimental workflow. The passive NNS and the active NNS were then used to measure the secondary neutron fluence rate spectra produced by a Varian TrueBeam T M STx linac under identical experimental conditions. We found that the spectrum obtained using the active NNS agreed well with that obtained using the passive NNS within uncertainties. This serves as validation of the use of the current-mode of the active NNS in the high neutron fluence rate conditions encountered in radiotherapy.

Abstract: Neutron spectrometry and subsequent dosimetry measurements were undertaken at the McMaster Nuclear Reactor (MNR) and AECL Chalk River National Research Universal (NRU) Reactor. The instruments used were a Bonner sphere spectrometer (BSS), a cylindrical nested neutron spectrometer (NNS) and a commercially available rotational proton recoil spectrometer. The purposes of these measurements were to: (1) compare the results obtained by three different neutron measuring instruments and (2) quantify neutron fields of interest. The results showed vastly different neutron spectral shapes for the two different reactors. This is not surprising, considering the type of the reactors and the locations where the measurements were performed. MNR is a heavily shielded light water moderated reactor, while NRU is a heavy water moderated reactor. The measurements at MNR were taken at the base of the reactor pool, where a large amount of water and concrete shielding is present, while measurements at NRU were taken at the top of the reactor (TOR) plate, where there is only heavy water and steel between the reactor core and the measuring instrument. As a result, a large component of the thermal neutron fluence was measured at MNR, while a negligible amount of thermal neutrons was measured at NRU. The neutron ambient dose rates at NRU TOR were measured to be between 0.03 and 0.06 mSv h⁻¹, while at MNR, these values were between 0.07 and 2.8 mSv h⁻¹ inside the beam port and <0.2 mSv h⁻¹ between two operating beam ports. The conservative uncertainty of these values is 15 %. The conservative uncertainty of the measured integral neutron fluence is 5 %. It was also found that BSS over-responded slightly due to a non-calibrated response matrix.